Beam forming lens



Ema/r: HUUM FIG. 2.

LIQUID OR SOLID GAS,

THEODORE C. CHESTON INVENTOR BY M, W

ATTORNEY United States Patent M 3,230,536 BEAlW FORMING LENS Theodore C.Cheston, Bethesda, Md., assignor to the United States of America asrepresented by the Secretary of the Navy Filed Apr. 13, 1962, Ser. No.187,441 11 Claims. (Cl. 343-754 This invention relates in general tospherical antennas and more particularly to a spherical antenna systemutilizing a lens as a beam forming device. More specifically theinvention relates to a dielectric lens having the characteristics of theLuneberg lens described by R. K. Luneberg in Mathematical Theory ofOptics, Brown University Advanced Instruction and Research in Mechanics,Providence, Rhode Island, summer of 1944.

In a tracking radar system, a highly directional radiation pattern isrequired to obtain a high degree of sensitivity. Rather than to alloweven a unidirectional radiation pattern, the transmitted energy of suchsystems is confined to a beam. Furthermore, an added requirement of suchradar systems is the necessity for providing an omnidirectional scanningcapacity.

The need for a highly directional signal could be satisfied through theuse of a spherical antenna having the properties of the aforesaidLuneberg lens. However, of practical necessity, the power level in sucha beam forming device must be far below that required for long rangeoperation. Therefore, it is necessary to provide a separate beam formingdevice in addition to the spherical antenna. This device has the beambending properties of the Luneberg lens, but additionally has manycollecting elements distributed about its surface. The signal to beradiated is injected into the spherically shaped lens at one point onits surface and expands within the sphere from that point into manyindividual portions which are separately picked up by the collectingelements located about the opposite surface of the lens. The signalpicked up by each collecting element is first amplified and thentransferred to the antenna by transmission lines. All of the collectedsignals are then radiated into space providing a highly directionalconcentrated beam. In effect the signal derived by the lens and presentat its surface is divided into several component portions by the lensallowing amplification of each portion so as to preserve thecharacteristics of the beam for radiation from a separate sphericalsurface.

The antenna requirements for providing omidirectional scan are achievedthrough electronic switching. That is, the signal is injected atdifferent points on the surface of the sphere and will form a beam inthe direction dictated by the point of signal injection. Therefore, themechanical rotation of the normal radar antenna system is no longerrequired since the electronic switching performs the same function in amore advantageous way.

The beam bending property of the Luneberg lens is attributed to thedielectric material from which the lens is made. This dielectricmaterial has an index of refraction, N, which is functionally related tothe radius r, with in the lens, by the equation:

where r =the outer radius of the sphere. As is evident from the equationthis lens will comprise a dielectric filled sphere having a continuouslyvarying dielectric constant from its center to its outer surface.

Because of the great difficulty associated with the manufacture of adielectric lens having a constantly varying index of refraction, variousmethods have been proposed to simulate a continuously varying index ofrefraction. Examples of two such approximations are explained in3,230,536 Patented Jan. 18, 1966 US. Patents 2,849,713 to G. P.Robinson, Jr. and 2,943,- 358 to S. F. Hutchins et al.

Notwithstanding the various attempts to minimize the expense ofmanufacturing an approximation of the Luneberg lens, the expense hasremained high because of the close tolerances that must be achievedduring the manufacturing process.

One object of the present invention, therefore, is to provide asimplified lens for a radar transmitting antenna system.

Another object of the present invention is to provide a lens constructedof a homogeneous dielectric material having a uniform index ofrefraction.

A further object of the invention resides in the provission of a lenswhich is constructed in the form of a sphere and that has air as itsdielectric.

A still further object of the invention is to provide a lens for lowfrequency waves having a very high index of refraction with acomparative reduction in size.

Other objects and many of the attendant advantages of the presentinvention will be readily appreciated as the same becomes betterunderstood by reference to the following detailed description whenconsidered in connection with the accompanying drawings, wherein:

FIG. 1 is a schematic representation of the wave shaping characteristicsof a lens of the instant invention;

FIG. 2 illustrates an embodiment of the lens of the instant invention;

FIG. 3 shows a schematic representation of the lens in combination witha spherical antenna; and

FIG. 4 shows a schematic representation of the invention for a twodimensional scan where linear polarization may be used.

This invention may be used, for example, to replace the transmitter lensrequired by the system of John B. Garrison, described and illustrated inhis application Serial No. 20,231, filed April 5, 1960, and assigned tothe US. Government.

The formation of a beam having parallel rays from an omnidirectionalsignal injected at a point source on the surface of a Luneberg lens iscaused by the beam bending property of the lens which continuouslyadjusts the phase and amplitude of the individual portions of theinjected signal and provides the proper phase relationship between allof the portions of the signal on the opposite side of the lens from thepoint of signal injection so that radiation into space of a plane waveresults at its exit aperture. It has been discovered that a sphericallyshaped dielectric lens with an invariant dielectric constant of 3.5:10%approximates the beam forming properties of the aforesaid Luneberg lens.These invariant dielectric constant lenses perform well in sizes up toat least a diameter of 30 wavelengths and may be used in the same way asLuneberg lenses.

Referring to the schematic representation of the lens 2 shown in FIG. 1,the signal phase produced by the lens can be seen to be dependent solelyupon the electrical path length through the lens, as designated by thelength of a line 4 between a radar transmitting feed 6 and one of theradar collectors 8. The length of a line 4 determines the amount ofadjusted phase for that collector.

Each collector 8 receives a signal whose phase, relative to its phase atthe radar transmitting feed 6, is given by the equation:

where I =the relative phase at the collector, L=the physical path lengthbetween the collector and the feed,

e=lh6 assigned value of the dielectric constant of 3.5;

and 21r/)\=Zt well-known constant in optics.

If radiated from this surface, all of the individual signals wouldcombine to form a wave having an equiphase wave front at the exitaperture designated by line 10.

As long as the product L\/e in the aforementioned equation remainsconstant the path length, L, and the dielectric constant 6, may bevaried. An embodiment of the instant invention for use with loWfrequency waves is derived by increasing the value of 6. This isaccomplished by utilizing a solid dielectric material having arelatively high refractive index. The radius of the resulting lens willthen be reduced correspondingly. By reducing the value of e, a lens willresult of correspondingly larger dimensions. In such an embodiment evena liquid such as paraflin oil can be used and Will perform as well asthe solid dielectric material. If e is reduced to unity, it is possibleto use air as the dielectric material. Compensation for this change ismade by increasing the value of L by the factor of /3.5il%. Theresultant lens will then have a radius that is increased by a factor of\/3.5i10% and this will eliminate the need for a homogeneous dielectricmaterial other than air.

The above-mentioned equation assumes the form when written in terms ofphysical size, where R=the radius of the lens having a homogeneousdielectric material,

R =the radius of antenna used with the radar system,

K=the refractive index of the lens having a radius R; and

The lens 2 employed in the instant invention is shown in FIG. 2 whereina spherical mounting shell 14, formed of a high loss material such as ametal covered with iron oxide, is used to support a plurality of radartransmitting feeds 6 and radar collectors 8. There can be any reasonablenumber of radar transmitting feeds and radar collectors distributed onthe surface of the shell. The number is limited only by the physicalsize of the feeds themselves. The mounting shell 14 is supported on apedestal 16 or any other physical means of support which will notinterfere with the proper operation of the lens.

Referring to FIG. 3, there is seen a schematic representation of thelens 2 of the instant invention in combination with a spherical radarantenna 18 having, located on its surface, radiating elements 20 whichcorrespond to those on lens 2 and which are connected to theirrespective radar collectors 8 on the surface of the lens 2 by aplurality of transmission lines 22, all of which are of equal length.For the sake of clarity, the intervening power amplifiers are not shown.An equiphase wave front 10' is radiated from the surface of the antenna18.

Referring to FIG. 4, there is seen a disk shaped lens 24 enclosed by amounting shell 26 having a radar transmitting feed 6 and a plurality ofradar collectors 8 mounted thereon. This embodiment of the inventionshows the adaptation of the principles of the invention to a cylindricallens 24. The transmission lines 22 are all of equal length and transposethe signals collected by the collectors 8 to corresponding radiators 20mounted on a similarly oriented radiating disk 28. This configurationproduces a fan shaped beam of radiation having its width in the plane atright angles to the plane of the disk 28 and is defined by the size ofthe radiators 20 in that plane. Linear polarization is permissible forthis configuration.

Obviously, many modifications and variations of the present inventionare possible in the light of the above teachings. It is therefore to beunderstood that within the scope of the appended claims the inventionmay be practiced otherwise than as specifically described.

What is claimed is:

1. In combination with a radar system including a spherical antenna, aspherical lens for focusing and refracting waves having a givenwavelength, said lens comprising, a. shell, a plurality of radartransmitting feeds distributed adjacent the surface of said lens andmounted in said shell for injecting a wave into said lens from anyselected feed, a plurality of radar collector means uniformlydistributed over the remaining portion of said lens diametricallyopposite said plurality of feeds, said plurality of collectors servingto collect a wave emergent from said lens, and said lens beingconstructed of a homogeneous dielectric material having a uniformrefractive index.

2. In combination with a radar system including a spherical antenna, alens as claimed in claim 1, wherein the homogeneous dielectric materialis a gas.

3. In combination with a radar system including a spherical antennahaving a radius R a lens as claimed in claim 1, wherein the dielectricmaterial has a refractive index K greater than 1 and the radius R of thelens is functionally related to the radius R according to therelationship 4. In combination with a radar system including a sphericalantenna having a radius R a lens as claimed in claim 1, wherein thedielectric material has a refractive index K greater than 1 and theradius R of the lens is functionally related to the radius R accordingto the relationship where Z=3.5i10%.

5. In combination with a radar system including a spherical antennahaving a radius R a lens as claimed in claim 1, wherein the dielectricis air and the radius R of the lens is functionally related to theradius R according to the relationship R:R 3.5.

6. In combination with a radar system including a spherical antenna, alens as claimed in claim 1, wherein the homogeneous dielectric materialis a liquid.

7. In combination with a radar system including a spherical antenna, alens as claimed in claim 1, wherein the homogeneous dielectric materialis a solid.

8. In combination with a radar system including a disk shaped antenna, adisk shaped lens for focusing and retracting waves having a given wavelength, said lens comprising, a disk shaped shell, a plurality of radartransmitting feeds distributed adjacent the surface of said lens andmounted on said shell for injecting a wave into said lens from anyselected feed, a plurality of radar collector means uniformlydistributed over the remaining portion of said lens diametricallyopposite said plurality of feeds, said plurality of collectors servingto collect a wave emergent from said lens, and said lens beingconstructed of a homogeneous dielectric material having a uniformrefractive index.

9. In combination with a radar system including a disk shaped antenna, alens as claimed in claim 8, wherein the radius of the disk shapedantenna is designated as R the dielectric material has a refractiveindex K greater than 1, and the radius R of the lens is functionallyrelated to the radius R according to the relationship 10. In combinationwith a radar system including a disk shaped antenna, a lens as claimedin claim 8, where- 5 in the radius of the disk shaped antenna isdesignated as R the dielectric material has a refractive index K greaterthan 1, and the radius R of the lens is functionally related to theradius R according to the relationship Where Z=3.5i10%.

11. In combination with a radar system including a disk shaped antenna,a lens as claimed in claim 8, where- 10 in the radius of the disk shapedantenna is designated as R the lens has air as its dielectric and theradius R of 6 the lens is functionally related to the radius R accordingto the relationship R=R., /3T

References Cited by the Examiner UNITED STATES PATENTS 2,566,703 9/1951Iams 343753 3,145,382 8/1964 Cuming et a1 343-911 X CHESTER L. JUSTUS,Primary Examiner.

ELI LIEBERMAN, Acting Primary Examiner.

M. KRAUS, Assistant Examiner.

1. IN COMBINATION WITH A RADAR SYSTEM INCLUDING A SPHERICAL ANTENNA, ASPHERICAL LENS FOR FOCUSING AND REFRACTING WAVES HAVING A GIVENWAVELENGTH, SAID LENS COMPRISING A SHELL, A PLURALITY OF RADARTRANSMITTING FEEDS DISTRIBUTED ADJACENT THE SURFACE OF SAID LENS ANDMOUNTED IN SAID SHELL FOR INJECTING A WAVE INTO SAID LENS FROM ANYSELECTED FEED, A PLURALITY OF RADAR COLLECTOR MEANS UNIFORMLYDISTRIBUTED OVER THE REMAINING PORTION OF SAID LENS DIAMETRICALLYOPPOSITE SAID PLURALITY OF FEEDS, SAID PLURALITY OF COLLECTORS SERVINGTO COLLECT A WAVE EMERGENT FROM SAID LENS, AND SAID LENS BEINGCONSTRUCTED OF A HOMOGENEOUS DIELECTRIC MATERIAL HAVING A UNIFORMREFRACTIVE INDEX.